The overall objective of this project is to define the cellular and molecular events which underlie the differentiation of B cells into IgA-bearing and secreting B cells. In previous studies, it was shown that T cell clones derived from Peyer's patches had the capacity to induce IgM-bearing B cells (in the presence of LPS) to differentiate into IgA-bearing B cells. The present series of studies were undertaken to define the molecular events which accompany T cell-induced B cell switches, particularly IgM IgA switches. Initially, we developed a series of T cell hybridomas from Peyer's patches and spleen by fusing Lyt-1+ T cells with BW5147 mouse tumor cells. We then determined the capacity of selected hybridomas to cause differentiation of various clonal B cell lines. We found that the pre-B cell line, 70 Z/3, exhibited only small amounts of surface IgM and no surface IgG or IgA when carried in culture; however, following stimulation with LPS, surface IgM increased dramatically. In contrast, treatment of 70 Z/3 B cells with LPS in the presence of any of several (but not all) PP-derived T cell hybridomas as well as several spleen-derived T cell hybridomas, substantial amounts of IgG (as well as IgM) appeared on the cell surface. In addition, in the case of one PP-derived T cell hybridoma, IgA also appeared on the cell surface. Similar results (i.e., appearance of surface IgG) was obtained when 70 Z/3 B cells were co-cultured with selected T cell hybridomas and IL-1 alone or anti-sepharose beads plus IL-1. Examination of cells for their content of Ig specific m-RNA disclosed that LPS-stimulated 70 Z/3 B cells, but not unstimulated 70 Z/3 B cells contain gamma and alpha-mRNA but do not show rearrangement of gamma and alpha heavy chain DNA segments. These results show that T cells derived from the Peyer's patch can induce changes in Ig-class expression in the pre-B cell line 70 Z/3; these findings are consistent with either true Ig switches or, alternatively, increased transcription of pre-existent IgG m-RNA.